1. Field of the Invention
The present invention concerns an antenna system with electronic scanning and digital beam forming and, notably, a way of achieving precise pointing in a wide frequency band, over a very extensive range of angles.
In these antennas, a fixed array of a very great number of elementary antennas is used. Each of these elementary antennas receives (or transmits) an elementary signal, and the combination of the different elementary signals corresponds to the wave to be received (or transmitted).
Electronic scanning consists in receiving (or transmitting) a wave that is not oriented in the same direction as the array, for example a wave with a direction of propagation that forms an elevation angle and/or an azimuth angle with the axis of the array.
To carry out this electronic scanning, it is necessary to apply a temporal or time delay to the signal received (or transmitted) by each of the elementary antennas, this temporal delay corresponding to the increase in the path of propagation introduced by the inclination of the pointing direction with respect to the axis of the array. This is illustrated in FIGS. 1 and 2, where the reference 1 designates each of the elementary antennas, P the plane of the array (for the clarity of the description, it shall be assumed that it is a linear plane array) and P' the plane of the wave to be received or transmitted in the pointing direction .delta.. It is thus seen that, for each elementary antenna 1, it is necessary to apply a delay .DELTA.t.sub.1, .DELTA.t.sub.2 . . . .DELTA.t.sub.n that is different from one antenna to another.
2. Description of the Prior Art
Essentially two techniques have been proposed to achieve this gradation of temporal delays.
The first technique consists in making an approximation of the delay by phase shifting the received wave.
This technique is easy to implement because it requires only purely electronic means (a phase shifter circuit placed in the active module associated with each of the elementary antennas). Furthermore, the phase shifts can be adjusted swiftly and with adequate quantification.
Despite its flexibility of use, this technique can be used, unfortunately, only for angle variations that are smal relatively to the dimensions of the array (the phase shift is only an approximation of the temporal delay) or for a very narrow frequency band.
In effect, with respect to the latter point, since the phase relationship depends on the frequency, a phenomenon of frequency spread is observed if the operation is outside a narrow frequency band. This phenomenon of frequency spread is similar to that of the chromatic aberrations encountered in optics in the case of Fresnel lenses and prisms for example.
In other words, with pointing done by means of phase shifters, the sensitivity of the pointing to the frequency means that the operation is very soon limited by the very small instantaneous band in which the pointing precision, provided by the number of elements of the antenna and the fineness of control of the phases, is obtained.
This is why, when the spectrum of the operating frequencies of the antenna has to be wide, notably if high resolution in distance is sought, it becomes necessary to abandon the technique of approximation by phase shifting and to introduce a real pure delay.
To implement this second technique of pure delay (to which the system of the invention is related), up till now propagation delay lines have been used. These propagation delay lines are either radioelectric (coaxial lines) or optical (optic fibers, after electro-optical conversion).
Each reception channel thus has a battery of delay lines. For each direction aimed at, a switching is done, for each channel, of that line which makes to possible to obtain the delay corresponding to the gradation of delays.
Since this technique introduces a pure delay and no longer an approximation of a delay, it removes the above-mentioned faults of frequency spread and therefore permits operation over a very wide band and for a large-sized array.
However, it has drawbacks, notably in its practical implementation: in effect, since the procedure is carried out by switching operations, the delay cannot be made to vary continuously, and it is therefore necessary to provide for as many lines as there are discrete directions in which it is sought to point the antenna. This leads to having a total number of delay lines, for the entire array, that is equal to the desired number of discrete pointing directions, multiplied by the number of elementary antennas of the array. It will easily be understood that, for an antenna with high angle resolution, for which it is sought to make maximum use of its potential precision, the number of delay lines needed is prohibitively great.
In addition, the (electrical or optical) switching of the delay lines implies a non-negligible response time that introduces a certain degree of slowness into the "reprogramming" of the antenna array (i.e. the modification of its pointing and of its relationship of illumination).
If a continuous coverage of the pointing directions is desired, the two above-mentioned techniques have to be combined, and the pointing then results from a main pointing (choice of a direction) by pure delay combined with a secondary pointing (fine pointing in the chosen direction) by phase shifter.
However, this combined approach is complicated to make and it is difficult to control the pointing because of the superimposition of two different means, which therefore makes it particularly costly.
One of the aims of the present invention is to propose a new pointing method that overcomes the drawbacks of both of the two above-mentioned techniques while at the same time being very simple and inexpensive to implement, and providing a possibility of varying the pointing direction over a very wide range, almost continuously and without any phenomenon of frequency spread.